Recent Developments in Analytical Science - Raman Spectroscopy

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  • Published: Jul 18, 2016
  • Channels: Electrophoresis / HPLC / Proteomics & Genomics / Ion Chromatography / Gas Chromatography / Proteomics / Raman / X-ray Spectrometry / Infrared Spectroscopy / NMR Knowledge Base / MRI Spectroscopy / Atomic / Base Peak

Raman Spectroscopy

Molecular spectroscopy covers a range of techniques in which the target compounds are analysed intact, relying on a particular property as the basis of the analytical method. Like mass spectrometry, this too is a growing sector. A recent market report confirmed that NMR and UV-vis spectroscopy take the top two places in market share but suggested that Raman spectroscopy will grow at the fastest rate up to 2018. It is a staple part of the analytical chemist’s diet but recent advances have kept the technique fresh and extended its scope. The popularity of the technique rests partly on its ability to analyse solids, liquids and gases.

Conventional Raman spectroscopy has been exploited in many areas, including the medical world. A point-of-care device has been designed to diagnose gout and pseudogout in patients by looking for monosodium urate monohydrate crystals and calcium pyrophosphate dehydrate crystals, respectively, in synovial fluid.18 The agreement with compensated polarised light microscopy, the clinical standard, is very good and the relatively low cost of the components make it an attractive alternative.

The availability of portable Raman instruments has been a boon in the analysis of art and archaeological objects, highlighted by the biennial International Congress on the Application of Raman Spectroscopy in Art and Archaeology. The proceedings of the seventh congress reveal the extent to which art historians turn to the technique with applications to gilded plasterwork, old gemstones, historical paintings, murals, pigments and monuments.19

In Raman spectroscopy, the proportion of light that is scattered is very low, yielding relatively weak signals, but the development of surface-enhanced Raman spectroscopy (SERS) delivered enhancement factors as high as 1014. The key to the amplification is the use of plasmonic materials such as silver, gold and nanoparticles. As long as the samples are in close proximity, signal amplification will occur.20 Combined with the non-destructive nature of the technique, SERS has become important in a range of applications.

Portable, commercial SERS sensors have increased the effectiveness of SERS in the field, where they have been used as detectors for chemical warfare agents, drugs of abuse in roadside tests, food contaminants and pesticides. For instance, a handheld SERS device was used to detect femtomole amounts of the nerve gases VX and tabun.21 A number of testing kits have been designed so that first responders can test drivers for illicit drugs in a similar way to the rapid alcohol breath tests that are routinely carried out. Amphetamines can be detected in urine with a portable SERS kit that requires just three minutes for extraction before analysis on gold nanorods arrays.22 The more common biofluid for drug testing is saliva and one low-cost handheld system has been demonstrated for the detection of cannabis and cocaine.23 On-the-spot testing kits like these will help to keep members of the general public safe from affected drivers.

Banned substances in foods can be routinely tested using SERS. The detection of melamine, Sudan dyes, hormones, fungicides and antibiotics have all been demonstrated.24 Despite their success, handheld SERS devices are not designed for measuring components and this remain one of the main challenges. Some studies have shown that semi-quantitative analysis is possible but the need for specialised software is clear.

A version of SERS with nanometre resolution is tip-enhanced Raman scattering (TERS) which combines the best features of SERS and atomic force microscopy. “It is one of the few techniques that provide chemical and structural information at ambient condition at high spatial resolution,” said scientists at the National Physical Laboratory. When a plasmonic metal-coated tip on the scanning probe microscope is irradiated with the laser beam, signal enhancement is limited to the nanoscale area of the sample under the tip, giving spatial resolution below 10 nm.

TERS remains a technique for experienced scientists and has produced some remarkable results for single DNA strands.25 Individual carbon nanotubes, pathogens, catalytic surfaces and single molecules have also been studied. TERS has been limited by difficulties in reproducible coating of the tips which are being overcome. So, with the advent of instrument automation and the production of reference standards, it has the potential to become a routine analytical technique.

Raman spectroscopy is also entering the realm of medical imaging, due to improvements in laser technology which eliminate much of the interfering background scattering. In one set up, the enhanced signals combined with new data processing software allowed the imaging of vitamin E on skin and in breast cancer tissue in situ. The microsecond timescale is compatible with in vivo testing on patients and has the advantage over other imaging methods that no dyes or labels are required to produce the signals.26

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